In the oil and natural gas exploration and development process, low-pressure and leaky formations are often encountered. During well cementing operation in a low-pressure and leaky formation, a problem of cement slurry leakage and inadequate return height is often encountered. To prevent leakage during well cementation, foamed cement slurry is often used for well cementation.
Foamed cement slurry refers to a low-density three-phase viscous fluid containing stable and evenly distributed foams, prepared by charging a gas into cement slurry and improving foam stability by means of a surfactant or macromolecular substance. Foamed cement slurry has advantages such as low density, low permeability, low fluid loss, low heat conductivity, high compressibility, and high strength, etc., can effectively solve the problem of leakage during well cementation in low-pressure and leaky formations, and is used more and more widely.
There are mainly two methods for preparing foamed cement in the field: one method is mechanical aeration method, and the other method is chemical aeration method. The mechanical aeration method requires more devices, involves a complex process, and requires elaborate design and automatic computer control; therefore, it is difficult to apply that method in the field, and the cost is high. In contrast, the chemical aeration method requires less devices and involves a less complex process flow; specifically, a chemical substance that can produce a gas (mainly a chemical substance that produces nitrogen gas) is directly added into the oil well cement material or water; hence, that method has advantages such as simple operation and low cost, and is the future developing trend of foamed cement.
There are many substances that can produce nitrogen gas through chemical reaction. For example, sodium nitrite and ammonium chloride can react to produce nitrogen gas, while releasing a great deal of heat. That reaction is applicable to heat-generating hydro-fracturing fluids and thermal chemical oil recovery, etc. However, sodium nitrite and ammonium chloride can react to produce nitrogen gas only in an acidic environment, but cement slurry is an alkaline medium environment, in which sodium nitrite and ammonium chloride can't produce nitrogen gas.
In addition, azo substances, such as azobisformamide, azodiisobutyronitrile, dinitrosopentamethylene tetraamine, 4,4′-dioxylbisbenzenesulfonyl hydrazide, p-toluenesulfon hydrazide, 5-phenyl tetrazole, and trihydrazinotriazine, etc., can produce nitrogen gas when they are decomposed; however, those substances can be decomposed and produce nitrogen gas when the temperature reaches their decomposition temperature under a heating condition. As a result, they are not suitable use in the surface preparation environment and under the conditions for preparing foamed cement slurry; in addition, those substances are flammable and explosive, involving some potential safety hazards.
Furthermore, when the foamed system is generated, the energy of the system will increase as the total area of the system increases. Therefore, such a system belongs to a thermodynamically unstable system and tends to evolve towards the direction of decreased energy and reduced total area, i.e., the foams tend to burst. To prepare a stable foamed cement slurry system, often a foam stabilizer has to be added into the cement slurry. Most foam stabilizers are surfactant substances, which can form a directional alignment on the surface of solution (usually water), and thereby remarkably decreases the surface tension (or interfacial tension) of the solvent, modifies the mechanical strength and viscosity of the foam film, and improves the stability of the foamed cement slurry. At present, the foam stabilizers for foamed cement slurry are mainly sulfonate anionic surfactants, such as alkylbenzene sulfonates, which have good salinity tolerance and heat resistance properties. Though foamed cement slurry mixed with such a foam stabilizer can produce foams easily, it is difficult to stabilize the foams, i.e., the foam stability is poor. Consequently, it is unable to prepare foamed cement slurry at lower density; in addition, the prepared foamed hardened cement has shortcomings such as uneven pore structures, high permeability, and low compressive strength, etc. Moreover, sodium alkyl benzene sulfonate foam stabilizers have poor biological degradability, and may cause severe environmental pollution. In recent years, people set more attention to the biological effect, toxicological effect and safety of surfactants, and gave more and more favor to the research and application of environment-friendly foam stabilizers.
A technical research on chemical aeration method for preparing foamed cement slurry is reported recently, and chemical gas generating materials FCA and FCB, which have chemical reactions in cement slurry to produce nitrogen gas, are developed, and a foam stabilizer FCF is added, to prepare low-density foamed cement slurry. However, with that method, a hollow micro-sphere material is utilized first to decrease the density of cement slurry to a certain range, and then the chemical gas generating materials are utilized to produce nitrogen gas, so as to further decrease the density of cement slurry to 1.20 g/cm3. Though the chemical aeration method for preparing low-density foamed cement slurry mentioned in the above literature can utilize chemical gas generating materials to prepare low-density foamed cement slurry, the density of the cement slurry can only be decreased to 1.20 g/cm3 at the most; namely, it is unable to obtain foamed cement slurry at lower density; furthermore, the compressive strength of the foamed hardened cement is low, not enough to meet the requirement for well cementation of oil and gas wells.